|“||On the first of June, around 9:30 p.m. you entered the school pool. Marina was there. You had an increasingly heated argument...until you picked up the trophy, and hit her in the back of the head with it. She didn't die instantly, which means you left her there, bleeding to death.||”|
|— Inspectora, in "Assilah"|
When Marina learnt that Nano was in danger if didn't pay the thugs who protected him in prison, she promised to get him the money. Marina advised Nano to a steal Teo's collection of expensive watches.
Marina and Nano planned to run away with the money from the stolen watches after learning that she was pregnant. In turn, one of the watches contained sensitive information that would incriminate Carla and Marina's fathers of corruption and embezzlement of funds.
At the end of the school party, while alone by the pool, Marina was confronted by Polo, who tried to force her to give him the stolen watch in an attempt to please Carla. When Marina refused and aggravated the situation, Polo grabbed the trophy and struck the back of her head, killing her. Polo recovered the watch. With the help of Carla and, more reluctantly, Christian, Polo managed to hide his involvement in Marina's murder, until Samuel discovered in the truth in Season 2.
- Polo: Polo is the real culprit in the murder, and kills Marina accidentally in a fit of rage when arguing with her over returning Carla's father's watch — a watch containing incriminating evidence of him and Marina's father.
- Carla: Carla was Polo's accomplice and the second most culprit. In an attempt to save her ex-boyfriend, Polo, she tells him to destroy the evidence and helps him clean up the blood, including forcing Christian to give Polo a change of shirt. She eventually crumbles under the pressure and reveals everything to the inspector.
- Christian: Christian is aware of the murder, and Polo's involvement, however does not reveal anything to the police. This is partly due to his love for Carla, but she also manipulates him into keeping it a secret.
- Teo: Teo removes Christian from the equation when he tries to confess Polo's guilt to the police. He causes him a fatal accident, forcing him to leave the country to seek medical help.
- Ander: Ander finds out about Polo's involvement and initially wishes to tell Guzmán, however is convinced not to. He keeps this a secret despite it destroying his relationship with Guzmán.
- Cayetana: When Carla reveals everything to the authorities, Cayetana tells Polo that everyone deserves a second chance. She hides the trophy — the murder weapon — in her house in an effort to remove evidence and free Polo; she does this because she is in love with him. After Polo dies, she gives the trophy anonymously to the police, thus closing the cause of Marina's murder.
- Andrea and Begoña : Polo reveals to his mothers at the dinner table that he murdered Marina and would turned himself in to the police the next day, but dies the same night at the disco. Later, when Cayetana gave the trophy to the police anonymously, his mothers confessed that before their son passed away, he confessed that he had murdered Marina.